In pressurized-water nuclear reactors, the vessel enclosing the core of the reactor has a head with a substantially hemispherical shape having openings in which followers are fastened enabling the passage of the control rods of the reactor and of instrumentation columns such as thermocouple columns; a set of thermocouples enabling the temperature of the cooling fluid to be measured at the outlet of the assemblies of the core of the reactor is arranged in each of the thermocouple columns.
Each of the followers has a part projecting beneath the head ensuring the guidance of the thermocouple column and a part projecting above the head having means for connecting a tubular bearing and sealing unit of the thermocouple column which may be fastened in the extension of the follower.
Inside the bearing unit fastened to the follower, a shoulder is provided against which part of the thermocouple column may bear, a sealing strip being placed therebetween.
The thermocouple column which traverses the bores of the bearing unit and of the follower, situated in each other's extension, has an end which engages with a pulling device resting on the end of the bearing unit.
By virtue of the pulling device, the thermocouple column may be displaced between a low position where its sealing surface is at a distance from the shoulder of the corresponding bearing unit and a high position where the sealing surface of the thermocouple column is applied against the shoulder with a degree of contact pressure which ensures sealing.
The bearing unit of the thermocouple column generally has two parts placed in each other's extension which are assembled, a sealing strip being placed therebetween, and held in place in their assembled position by a clamping bracket engaged on corresponding tapered areas of the two parts of the bearing unit.
The lower part of the bearing unit is connected to the follower by its joining means which generally consists of a threaded part onto which is engaged a corresponding tapped bore of the lower part of the bearing unit.
The joint between the follower and the lower part of the bearing unit is completed by welding two circular seams placed to coincide in order to ensure the sealing of the screwed joint.
The upper end part of the bearing unit including the sealing area of the thermocouple column has an upper end surface against which the pulling device of the instrumentation column bears.
When it is desired to raise the vessel head, after having performed the depressurization, the tensile force exerted on the thermocouple column is relaxed and the two parts are separated from the bearing unit by disassembling the clamping brackets.
The pulling device on the thermocouple column may be separated from the latter, enabling the upper part of the bearing unit to slide in order to separate it from the thermocouple column.
The thermocouple column may then be removed completely from the follower and from the lower part of the bearing unit.
This procedure is also necessary when it is desired to change the sealing strip of a thermocouple column.
These various operations take a relatively long time to perform and may even prove impossible when the clamping pieces seize up.